Method of testing plasticity



March 22, 1938 J. H. DILLON ET AL A 2,112,190

METHOD OF TESTING PLASTICITY l Filed Dec. l4, 1933 BY y. @QWBM ATTO RN EYS l, Patented Mar. 22, 1938 METHD @F TESTING PMSTIICITY .lohn H. Dillon, on, and Norris `lollnstoiua Cuyahoga Falls, Ohio, assignors to '111e Firestone Tire da Rubber Company, Akron, Ohio, a

corporation of Ohio Application December 14, 1933, Serial No. 102,336

Claims.

This invention relates to methods of .testing the plasticity of plastic materials, and more especially it relates to improvements in the methods of testing plasticity by extruding the plastic ma- 5 terial.

The invention is of primary utility in the controlling of plasticity in production stocks and to aid in developing new stocks which may be extruded satisfactorily. It has been found that plasticity tests heretofore made, both by the compression and the extrusion methods, fail to correlate with factory extrusion machines, due in large part 4to the fact that said methods employ much lower rates of shear'than obtains in the usual existing extrusion machines.

The chief object of the invention is to obtain a method of plasticity testing that will give accurate results, and which results correlate with factory extrusion methods. Other objects will be manifest.

Briefly stated, the invention consists in 'determining the rate of extrusion for a given pressure and temperature through an orifice of determinate size and shape. From these results it is possible to obtain the rate of shear, which may be considered a criterion of ow conditions suitable for comparative purposes. The results also may be used for determining the two quantities necessary to define the plastic state of a material, namely, the mobility and the yield stress.

Apparatus for the practice of applicants invention is shown in the accompanying drawing of which;

Figure 1 is acentral vertical section of apparatus adapted to carry out the invention in inoperative position, and a test piece of plastic rubber composition therein;

Figure 2 is a section of the apparatus shown in Figure 1, in the initial phase of operation; and

Figure 3 is a view similar to Figures 1 and 2, on a larger scale, showing the apparatus in the final phase of operation.

Referring to the drawing, l0 is, a stationary platen and ll is the vertically `movable platen of a suitable power press. Mounted upon the bottom of the stationary platen Ill is a box-like heatinsulating jacket .l2 of bakelite or the like, and

mounted in said jacket is an upper die i3 thatV is formed with a downwardly extending annular ange lll defining a cup-shaped concavity I5. The die member I3 is heatedto a determinate temperature by electric heating elements I6, I6 mounted therein, and a thermometer il may be provided for indicating said temperature. A vertical, reciprocable plunger I8 extends through a stuffing box lil in the upper platen lll and through an aperture 25 in the jacket l2, thelower end portion of said plunger being sildably mounted in an aperture 2l in the die member 13.4 The plunger is coaxial with relation to cavity l5, and its lower end is ush with the Surface of said cavity. Vent ducts 22 extending from aperture 2l] to the atmosphere are formed in the jacket l2.

Mounted upon the top of the movable, lower platen li is an insulating jacket 2li, and mounted in said jacket is a die block 25. The latter is formed with a central aperture 26 that is aligned with a central aperture 2l in the jacket 21 and a somewhat larger aperture 28 in the platen Il. Threaded into aperture 26 in the die block 25 is a die 29 formed with an extrusionorice 30, the upper end of which is tapered, flared, or countersunk at an angle of 45. The upper face of the die block is formed with an annular groove 3l that is concentric with the axis of die orifice `3|), the inner.; edge of said groove being formed with a bevel or chamfer as shown. The groove :il also is concentric with the flange ll of the upper die 13, the arrangement being such that when the lower platen Il is in elevated, operative position, the inner surface of the ilange il, at the lower edge thereof, will engage the beveled surface at the inner edge of groove 3l `with -a sealing ilt. The diel block 25 is heated to determinate temperature by electric heating elements 32 mounted 'therein,and a thermostatic control,'of which only the member 35 is shown, is provided for keeping the said die block at constant uniform temperature.

In the practice of the invention, test pieces of unvulcanized rubber composition 35 are prepared by preheating them for a suitable length of time, after which one of them is mounted `upon the die holder 25 as shown in Figure 1. The lower platen l I is then elevated to the operative position shown in Figure 2, with the result that the die holder 25 is moved into engagement with the upper die I3. Thus a local region of the test piece 35 is sheared therefrom and concurrently confined in the cavity l5 of the upper die i3, the excess of rubber in the cavity being forced through the orifice 30 ofthe die 29 in the form of an elongate cylindrical cord 36.

Air entrapped in the cavity l5 or occluded in the rubber escapes through orifice 20 and along the plunger i8, passing out through the ducts 22. The apparatus is allowed to remain in the position shown in Figure 3 for several minutes to allow the test piece confined in the cavity I5 to attain the proper temperature. 'I'he plunger I8 a determinate limit to the downward movement of the plunger I8, and the time required for the plunger to reach said limit is carefully ascertained, preferably by automatic timing mechanism (not shown). The extruded piece 36a is severed and measured before cooling, the measurement of length giving an inverse measure of the amount of recovery. The residual elasticity in the stock causes the extruded rubber strip a to expand in diameter upon emergence from the orifice. Since the volume of rubber is constant the measurement of length gives an inverse measure of diameter and consequently of elastic recovery. This completes the test and the platens may be separated and the remainder of the test piece in the apparatus removed.

Modification may be resorted to without departing from the spirit of the invention or the scope of the appended claims, which are not limited wholly to the exact procedure described.

What is claimed is:

1. The method oftesting plasticity which comprises preparing a test piece of plastic material oi determinate size by confining and compressing a piece oi' material of larger size and extruding the surplus material therefrom, and then displacing a determinate portion of the interior of the test piece by extruding it in the form of a strip of determinate cross-section. i

2. A method as dened in claim 1 including the step of determining the time required to eifect displacement of the material with determinate pressure.

3. A method as dened in claim 1 including the step of heating the test piece while it is confined.

4. The method of testing plasticity which comprises preparing a test piece of plastic material of determinate size by coning and compressing a piece of material of larger size in a manner that extrudes the surplus material therefrom, heating the confined material, displacing a determinate portion of the interior of the test piece by extruding it in the form of a strip of determinate cross section, and determining the time required to effect said displacement.

5. The method of testing plasticity which comprises shearing a quantity of plastic material from a larger size mass thereof and concurrently conning and compressing the sheared-oi piece to bring it to determinate size by extruding surplus material therefrom, extruding a determinate portion of said material, and determining the time required to eect the latter extrusion.

- JOHN H. DILLON.

NORRIS JOHNSTON. 

